1. Field of the Invention
The present invention is generally related to an offset (planograph) type rotary press, more particularly to an offset rotary press adapted for a color printing system including at least two printing sections which can sequentially and continuously print a paper-web travelling through the printing sections.
2. Description of the Prior Art
A typical conventional offset rotary press adapted for a color printing system is shown in FIG. 7. This conventional rotary press printing system comprises a plurality of printing sections P, each of which includes two pairs of a combination of a plate cylinder PC and a blanket cylinder BC. The blanket cylinders BC of each printing section P are vertically arranged to be in contact with each other. In this conventional printing system, four sets of the printing sections P are horizontally arranged in parallel. A paper web W is horizontally fed into the four sets so that the paper web W travels through the pressed space between four pairs of the blanket cylinders BC, BC to print both surfaces of the paper web W.
Another conventional offset rotary press for a color printing system is shown in FIG. 8 wherein four printing sets, each of which is composed of a plate cylinder PC and a blanket cylinder BC, are radially arranged about an impression cylinder IC as a common center cylinder. The blanket cylinder BC is in contact with the impression cylinder IC to form the printing section P. A paper web W travels around the circumference of the impression cylinder IC so that the paper web W is passed through the four printing sections P defined between the blanket cylinders BC and the impression cylinder IC to print one surface of the paper web W.
In recent years, many newspaper publishers have begun to print their publications with colored inks, thus demanding high performance color printing on many pages at a high speed and within a limited printing space.
According to such demands, other conventional offset rotary press color printing systems are shown in FIG. 9 and FIG. 10, wherein four printing sections P are vertically arranged. Each printing section P includes two sets of a blanket cylinder BC and a plate cylinder PC which are symmetrically arranged so as to bring the blanket cylinders BC into contact with each other. A paper web W vertically travels through the four printing sections P to print both surfaces of the paper web W in the same manner as the system described above. This type of printing system, for example, is shown in "IFRA Newspaper Techniques English Edition", pp. 64 to pp. 79; April, 1988, published by INCA-FIEJ Research Association.
The paper web to be printed is generally produced in such a manner that pulp fibers are mechanically out and broken into fine particles; dispersed in water; dehydrated and dried; and finally adhered by hydrogen-bond to form a paper sheet. Under moist conditions, each of the pulp fibers tends to extend a little less than 1 percent in its longitudinal direction and 20 to 30 percent in its radius direction. Thus the paper web is extended in its longitudinal and width directions by the dampening operation. Most of the pulp fibers of general mechanically produced paper web are orientated in the longitudinal direction of the paper web, so that the paper web is remarkably extended in its width direction.
In the offset type printing system employing the dampening procedure in the printing section, the paper web is caused to swell by the dampening water. Therefore, the printed pattern on the paper web is also deformed in response to the swell of the paper web. Even when the printing systems that include at least two continuous printing sections employ the dampening means applying water vapor to the paper web, the printed image formed at the first printing section does not correctly coincide with the image formed at the second and later printing sections. Accordingly, this will produce printed materials of poor quality.